Vehicles that operate on methanol fuel must meet governmental emission standards for nitrogen oxides, hydrocarbons, and carbon monoxide. Because of the lean-burn characteristics of methanol vehicles, the nitrogen oxide standards may be met without the use of catalytic converters. However, converters must be used to treat the exhaust gases in order to meet the carbon monoxide and hydrocarbon emission standards. Hydrocarbon emissions from methanol fueled vehicles are largely oxygenated organic compounds, of which unburned methanol is the principal component.
Efforts have been made by others to treat exhaust gases from a methanol fueled internal combustion engine. For instance, U.S. Pat. No. 4,304,761 issued to Yu Yao disclosed a method of oxidizing unburned methanol by passing the exhaust gases over a silver catalyst. Yu Yao disclosed that silver has a relatively low reaction temperature for a high percentage conversion of methanol to CO.sub.2 while producing no significant amount of undesirable byproducts such as formaldehyde. Yu Yao also disclosed that while platinum and palladium are active at low temperatures for converting unburned methanol, both platinum and palladium have an affinity for producing aldehydes.
The object of our invention is to simultaneously oxidize unburned methanol and carbon monoxide contained in methanol vehicle exhaust. It is known in catalysis that difficulties are frequently encountered when simultaneous chemical reactions are desired since certain chemical compounds can inhibit the reactions of other compounds. In an application such as automotive exhaust catalysis, a number of simultaneous chemical reactions must be accomplished at the same time, e.g., the oxidation of hydrocarbons, the oxidation of carbon monoxide, the reduction of nitric oxides, etc. Therefore, the inhibiting effects of the various species on the reactions of other species must be taken into consideration in such complex reactions.
We have discovered that the inhibiting effects of various species exist in the simultaneous oxidations of unburned methanol and carbon monoxide in methanol fueled vehicle exhaust. Specifically, carbon monoxide is adsorbed rapidly and strongly on a noble metal catalyst such as palladium at low temperatures thereby preventing the adsorption and reaction of unburned methanol. As a result, rapid oxidation of methanol can only be achieved at temperatures that are high enough to remove the carbon monoxide either by desorption or by reaction with oxygen.
It is therefore an object of the present invention to provide a method of treating exhaust gases from an internal combustion engine burning methanol fuel by simultaneously oxidizing the unburned methanol and carbon monoxide contained in such exhaust gases.
It is another object of the present invention to provide a method to efficiently treat exhaust gases from an internal combustion engine burning methanol fuel by simultaneously oxidizing the unburned methanol and carbon monoxide contained in such exhaust gases at low conversion temperatures.